1. Field of the Invention
The present invention relates to a process for sealing devices incorporating microstructures.
2. Description of the Related Art
As is known, the evolution of the techniques of micromachining of semiconductor materials, in particular silicon, has made it possible to obtain a wide range of devices based upon electromechanical structures having parts that are relatively mobile with respect to one another. By way of example, among said devices mention may be made of optical devices comprising tiltable micromirrors, micromotors, microactuators for fine positioning of read/write heads of magnetic disks, and sensors, such as pressure sensors and accelerometers both of a linear type and of a rotary type.
On the other hand, it is known that microelectromechanical structures, or microstructures, as they will be referred to hereinafter, are brittle and easily subject to mechanical failure, and consequently may be damaged both during fabrication and during use.
In particular, certain processing steps and subsequent steps of handling, transportation, assembly and use of the devices expose the microstructures to impurities that may penetrate between the moving parts and cause irreparable damages. For example, the step of cutting a semiconductor wafer comprising a plurality of devices in order to obtain dice each of which contains a single device is particularly critical. In fact, wafers are normally cut by means of a purely mechanical process, in which a saw cooled by a flow of water is used. Consequently, during the cutting operation a considerable amount of particles spread into the environment.
If the microstructure is not adequately protected either during the cutting step or during subsequent fabrication steps, dust, humidity or other contaminating agents may penetrate into the gaps that separate the relatively mobile parts of the microstructure itself. Clearly, the presence of external contaminating agents may cause a partial or total blockage or even failure of the microstructure.
To overcome the above mentioned problem, a known solution is encapsulating the microstructures in a protective structure before the wafer is cut. Normally, the protective structure comprises a second wafer of semiconductor material or of another material, such as glass, ceramics, or a plastic material, and is bonded on the wafer to be protected in such a way as to seal the gaps between the mobile parts, so rendering them inaccessible from outside. Once the wafer has been cut, each individual die comprises a respective portion of the protective structure, which in certain cases remains incorporated in the finished product, whereas in other cases it must be removed.
The above described solution, however, has some disadvantages. First, the use of a second semiconductor wafer or of a wafer made of another material for the sole purpose of sealing the microstructure involves a considerable cost. In the second place, the process is complex, since encapsulation requires numerous fabrication steps. For example, before bonding the wafer to be protected and the protective structure, it is necessary to prepare bonding areas; next, the protective structure must be removed or, alternatively, thinned out in order to reduce the overall dimensions of the finished device.
A further drawback is represented by the fact that the above mentioned solution can be effectively used only for certain types of devices, which, during use, can remain encapsulated (for instance, accelerometers). In other cases, instead, the microstructures interact directly with the outside environment and, consequently, the devices incorporating them must necessarily be opened during operation. For example, in microactuators for fine positioning of read/write heads of magnetic disks, the microstructure must enable the angular position of the head to be varied with respect to a supporting arm of a main actuator. For this purpose, a first part of the microstructure (stator) is fixed to the supporting arm, and a second part (rotor), which can turn with respect to the first part, is rigidly connected to the head. Precisely on account of the fact that in this case the function of the microactuator is to modify the positions of two bodies with respect to one another, clearly the microstructure must remain free and cannot be encapsulated. Likewise, also in optical devices provided with tiltable micromirrors, the microstructures, which must be reachable by the electromagnetic radiation coming from outside, cannot be encapsulated.
Hence, the processes according to the prior art are not suitable for protecting devices of the aforesaid type during their use. The said devices thus remain exposed to contaminating agents for a very prolonged period of time and, consequently, may easily get damaged.